whether the electron can be considered as a free particle, an elementary building block from which a new theory can be built. “In some theories, the system is strongly coupled from high-energy to low-energy – meaning that you cannot identify electrons. These theories are known as strongly-correlated electron theories,” explains Dr Pépin. This point of view has dominated over the last thirty years, but now Dr Pépin is developing an alternative theory dominated by the presence of an emerging symmetry, which is more complex than that for the standard theory of superconductivity. “The phenomenon of spontaneous symmetry breaking is a fundamental part of theoretical physics, from high-energy physics to condensed matter physics,” she explains. “In superconductivity, the spontaneous symmetry which breaks is a rotation symmetry which couples to light. It’s the same symmetry as that which describes electro-magnetism. This phenomenon of U(1) symmetry breaking is the same as for the Higgs Boson.”
Researchers plan to investigate the accuracy of this new theory through interactions and collaborations with experimental groups. These interactions are always easier when the theory under investigation is underpinned by a physical principle like a symmetry. “When a theory is controlled by a symmetry it is easier to make an effective model. For example, the Ginzburg-Landau theory was developed to describe superconductivity, before the underlying mechanism was found. And this worked well,” says Dr Pépin. “We also plan to test the accuracy of the theory through the prism of experimental data.” There are also plans to look at the building blocks of cuprates in experiments, which it is hoped will lead to new insights into the origin of superconductivity in these materials. “These compounds fascinate the research community, and experimentalists are working very hard to understand them. They have made a lot of progress, using techniques like Raman spectroscopy, time-resolved
When you accelerate an electric
charge, it should emit some electro-magnetic radiation, but this doesn’t happen in a superconducting material. In a conducting ring using superconducting materials, energy is not lost and the conduction of the current is eternal. This idea of perpetual motion was a dream of the Greek philosophers, and nature has given us a realisation of it The key idea in Dr Pépin’s research is that at low enough temperatures the physics of cuprate superconductors is still controlled by an emerging symmetry, but the symmetry group in this case is the first non-Abellian group, the SU(2) group. SU(2) symmetry originates in these compounds due to the competition between two types of order – superconductivity and a modulation of the electric charge. “These two competing orders are related by the emerging symmetry. So the complexity and the symmetry relies on the presence of a competing order in the phase diagram, infinitely coupled to superconductivity,” outlines Dr Pépin. “Certain topological defects might emerge from this coupling, and they could proliferate. This is something which we will take account of in the project.”
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X-rays, and scanning-tunnelling microscopy (STM), and huge sets of experimental data have been generated,” says Dr Pépin. “It’s very important for us to test the accuracy of our theory on experimental data. This is a very strong constraint.” The goal in this research is to understand how cuprates work, with the wider, longer-term objective of increasing the temperature below which they superconduct, to eventually reach room temperature; Dr Pépin says this would have far-reaching implications. “If we succeed in perfecting this phenomenon at room temperature then it will completely change our lives. We will be able to really conduct electricity without any energy loss. So it will completely change energy provision,” she stresses.
At a glance Full Project Title Charge orders, Magnetism and Pairings in High Temperature Superconductors (Champagne). Project Objectives In the quest for finding room temperature superconductors able to carry current without loss of energy, we propose that the physics of a class of materials, the cuprates, is governed close to the quantum vacuum, by an emerging SU(2) symmetry relating the superconducting state to the charge sector. A single gap equation describing a “non abelian” superconductor is tested to a wide range of experiments. Project Funding ERC-ADG-2015 - ERC Advanced Grant. Project Partners Xavier Montiel, Corentin Morice, and Debmalya Chakraborty. (Post Doctorates at the IPhT) Contact Details Dr Catherine Pépin Institut de Physique Théorique Orme des Merisiers CEA - Saclay F-91191 Gif-sur-Yvette - FRANCE T: +33 (0)1 69 08 72 18 E: cpepin@cea.fr W: http://ipht.cea.fr/Pisp/catherine.pepin/ index_fr.php X. Montiel, T. Kloss, C. Pépin, Phys. Rev. B 95, 104510 (2017) T. Kloss, X. Montiel, V. de Carvalho. H. Freire, C.Pépin, Rep. Prog. Phys., 79, 084507 (2016) K.B. Efetov, H. Meier, C.Pépin, Nat. Phys. 9, 442 (2013)
Dr Catherine Pépin
Dr Catherine Pépin is a Researcher in Theoretical Condensed Matter Physics at the Institut de Physique Théorique, based near Paris. Her main research interests are zero temperature phase transitions, in particular heavy fermion systems and high Tc superconductors.
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